1. Field of the Invention
The invention relates to doping semiconductor crystals and somewhat more particularly to a method and apparatus for positively doping semiconductor crystals via zone melting.
2. Prior Art
Generally, semiconductor crystalline rods are doped by depositing a semiconductor material from a suitable gaseous phase by thermal and/or pyrolitic decomposition of a gaseous compound of the semiconductor material onto a heated rod-like carrier member composed of the same semiconductor material. Select dopants are mixed with the gaseous compounds of the semiconductor material and are deposited on the carrier body. The semiconductor crystalline rods produced in this manner are polycrystalline and must be transformed into a monocrystalline state by a subsequent zone melting process. During such transformation the concentration of dopants often changes in an uncontrollable manner so that substantially higher dopant concentrations must be originally provided to enable a desired dopant concentration to exist in the final semiconductor rod, which may have to undergo a plurality of zone melting cycles.
Such prior art procedures are time-consuming and inaccurate. Apparatuses employed in carrying out such procedures are only capable of producing marginally satisfactory results and involve large capital and operating expenditures.
German Offenlegungsschrift 1,544,276 shows a process and apparatus for producing doped semiconductor crystalline rods wherein a gaseous dopant is guided to a melt zone on a semiconductor rod being processed inside an evacuated housing by a conduit which is open in the vicinity of the melt zone. With this type of process, the amount of dopant gas supplied to the melt zone is regulated with the aid of a valve located between the melt zone housing and the dopant supply housing and by the pressure within the dopant supply housing, which is maintained at a constant temperature. The dopants used in this process preferably are such compounds of boron and phosphorus which are easily handled and readily vaporized, for example, trimetric phosphorus nitrilochloride. A disadvantage of this process is that the valves which regulate the quantity of dopant do not operate accurately (generally, commercially available needle valves or pulse-controlled magnetic valves are used for this purpose). Accordingly, the reproducibility of a dopant concentration within a semiconductor rod produced in this manner is unreliable.
German Offenlegungsschrift 2,020,182 shows an apparatus for doping semiconductor rods by the crucible-free zone melting technique with an induction heating coil which annularly surrounds the rod and which includes a conduit therein through which a gaseous dopant is blown or carried directly into the molten zone with the aid of a carrier gas stream. This device enables the production of semiconductor rods having a relatively constant and adjustable dopant concentration with approximately just one zone melting cycle. The dopant concentration in a so-produced semiconductor rod is dependent upon the carrier gas flow. This apparatus is useful with readily vaporizable compounds of boron or phosphorus. However, the dopant concentration with a carrier gas stream may vary and no positive means accurately determining the amount of dopant introduced into the treated semiconductor rod is available.